Electric subsea installations and devices usually demand high standards regarding durability, long-term functionality and independence during operation. Another requirement is that stray inductance is kept very low in order to avoid overvoltage in the semiconductor switches and reduce the switching losses. Electric installations that need to be cooled during operation, such as subsea converters, require an autonomous and durable cooling of its components. It is known to use a dielectric liquid such as a for example oil as cooling fluid. In general electric subsea installations need to be pressurized by the dielectric fluid, thus said fluid, which is preferably a liquid, should be, at least almost, incompressible. The dielectric liquid is used as cooling medium and as electric insulation medium for insulating electrical components against each other and against a housing or tank.
Referring specifically to subsea converters there are some requirements that make the construction of a subsea converters quite difficult. Due to the high reliability that is usually required, and since it may be difficult to modify and provide access to personnel for maintenance for the subsea converter or for the electric installation from the outside, specialized personnel is normally hired. This is expensive and complicated and can lead to a long downtime of the electric converter. It is thus another requirement that the cooling system and the electric converter system is able to work autonomously and independent for a long period.
Cooling systems for electric equipment and specifically for electric subsea converters are used to cool down electric components, such as semiconductor modules, gate drives, control elements, connections and capacitors. These components generate heat that needs to be dissipated by the cooling system. The cooling system of subsea converters are usually designed in a simple manner avoiding any unnecessary parts and mechanisms. Generally it is desirable to have passive cooling systems, i.e., cooling systems without any driven or powered parts, such as for example pumps, to cool down the electric components. In some cases natural convection is used. Natural convection helps to avoid the use of a pump but it may have a low efficiency. Cooling by natural convection uses the heat exchange between the cooling liquid and the surrounding sea water to generate circulation within the cooling system and thus within the electric installation and the subsea converter, respectively. Usually some kind of a heat exchanger is used to transfer heat from the cooling liquid to the sea water.
In general, the outer sides of the housings of subsea converters, the heat exchangers, and any other parts that are in contact with the sea water should stay below a surface temperature of around 30° C. Thus even under full operating load the surfaces of the housing, the heat exchanger, and any other parts in contacts with the surroundings should be below said temperature value. If the temperature reaches values higher than around 30° C. sea life, such as algae, plankton, mussels, corals or the like, will grow on the outer side of the subsea converter and the heat exchanger respectively, which increases the thermal resistance of the housing and the heat exchanger of the subsea converter. As a result, sea life builds up on and clogs the outer surfaces of the subsea converter, thereby forming a thermal insulation layer that reduces the capability to dissipate heat from the affected surfaces. Since not all electric components of the subsea converter have the same heat flux that needs to be dissipated when in use, the sea life growth on the outer side of the subsea converter can be uneven which leads to different thermal insulation values on the surface of the subsea converter, making it even more difficult to predict and calculate the heat dissipation and to determine the optimal operating load of the electric converter.
Another problem that relates to the growth of sea life on the housing, specifically the growth of sea life on the heat exchanger, is the spacing of the fins of the heat exchanger. In order to avoid that the sea life completely blocks the circulation of sea water through the heat exchanger, the spacing between the fins that extend into the sea water, away from the subsea converter and its housing, needs to be chosen comparably large, thus reducing the efficiency of the heat exchanging process. In conventional electric modules the bus bar connections are quite long and sometimes cumbersome. However, it is an on-going requirement to keep the bus bar connections as short as possible since the material, such as copper, used for these connections is generally expensive, heavy and the longer the bus bar connections are the more electric losses appear, which is not desirable. Environmental laws or aspects may further require that the smallest amount of dielectric liquid is used, to avoid environmental damages in case of a failure.
DE 10 2011 006 988 A1 discloses a two-piece cell comprising a cooling body with an inlet and an outlet. The two-piece cell further comprises terminals X1 and X2.
EP 2 487 326 A1 discloses a subsea electronic system comprising pressure resistant enclosure with dielectric liquid and a heat generating electric component at least partially immersed in the dielectric liquid for cooling reasons.
In addition it is preferred that the electronic converter system satisfies multi-module and multilevel configurations, which allow an easy installation and removal of electric modules in order to provide a flexible and versatile electric converter system.
Hence there is a need to improve the design and configuration of an electric converter system, especially an electric subsea converter system, and also to improve the arrangement of electric components in electric converter systems.